1. Field of the Invention
The present invention relates to electronic circuitry, and, in particular, to power amplifiers for telecommunication applications.
2. Description of the Related Art
Wireless telecommunication systems typically operate over specific ranges of signal frequencies. For example, according to the U.S. PCS (Personal Communication System) standard for CDMA (code-division, multiple-access) communications, forward-link transmissions from base stations to mobile units occur within the 60-MHZ frequency range from 1930 MHZ to 1990 MHZ. (Reverse-link transmissions from the mobile units back to the base stations occur within a different frequency range.) The 60-MHZ forward-link transmit frequency range is divided into six frequency blocks: three 15-MHZ frequency blocks, each with eleven 1.25-MHZ frequency channels, and three 5-MHZ frequency blocks, each with three 1.25-MHZ frequency channels, for a total of 42 frequency channels, where each frequency channel can support up to, for example, 64 different CDMA code channels.
When configured in the field, a particular base station is assigned to operate within a particular PCS frequency block. In order to avoid having to design, build, and inventory six different types of base station transmitters (one for each different PCS frequency block), a single generic base station transmitter design is typically used for all frequency blocks. In that case, it is important that the generic base station transmitter operate satisfactorily at all frequencies within the 60-MHZ forward-link transmit frequency range.
FIG. 1 shows a block diagram of a conventional base station transmitter circuit 100 used in the forward-link transmitters of PCS telecommunication systems. Base station transmitter circuit 100 has an oscillator 102 that generates an RF (radio frequency) signal 104 having a frequency between 869 MHZ and 894 MHZ, and a transmit up-converter 106 that converts RF signal 104 into a PCS block signal 108 having a frequency corresponding to one of the PCS frequency blocks within the 1930-1990 MHZ PCS range (e.g., the center frequency for a particular 5-MHZ or 15-MHZ PCS frequency block). The particular PCS frequency block is specified by a digital control signal 120 received from alarm control board (ACB) 118.
PCS block signal 108 generated by up-converter 106 is then provided to high-power amplifier 110, which further amplifies the PCS block signal to generate an amplified PCS block signal 112 for subsequent transmitter processing (e.g., tuning to a particular PCS frequency channel within the PCS frequency block, data and code modulation, and transmission) (not shown in FIG. 1). In one implementation of base station transmitter circuit 100, high-power amplifier 110 is a Model No. 34874/EB500600-3 amplifier from MPD Technologies, Inc., a subsidiary of Microwave Power Devices, Inc., of Hauppauge, N.Y.
In addition, as indicated in FIG. 1, an amplifier alarm signal 114 is fed back to transmit up-converter 106 from high-power amplifier 110 to indicate the presence of an alarm condition within high-power amplifier 110. A transmit up-converter alarm signal 116 is also fed back to alarm control board 118 from transmit up-converter 106 to indicate the presence of an alarm condition within either transmit up-converter 106 or high-power amplifier 110.
FIG. 2 shows a block diagram of a conventional transmit up-converter 106 for base station transmitter circuit 100 of FIG. 1. As shown in FIG. 2, up-converter 106 receives RF signal 104 from oscillator 102 of FIG. 1 at 1-dB pad 202. Mixer 206 mixes the received RF signal with a mixing signal 224 from low-pass filter 222 to convert the received RF signal into a mixer output signal 204 having the desired PCS block frequency. Mixer output signal 204 is processed through potentiometer 208, 5-dB amplifier 210, 20-dB amplifier 212, and band-pass filter 214 to generate PCS block signal 108 for transmission to high-power amplifier 110 of FIG. 1.
Synthesizer 218 receives control signal 120 from ACB 118 and, in conjunction with 5-dB amplifier 220 and low-pass filter 222, converts a 15-MHZ local clock signal 216 into mixing signal 224 having a frequency appropriate to cause mixer 206 to generate mixer output signal 204 having the desired PCS block frequency specified by control signal 120. Synthesizer 218, which may comprise a phase-locked loop circuit or other controllable signal generator, generates a lock detect signal 226 to indicate when the desired frequency for mixing signal 224 has been achieved.
Logic circuits 228 (1) receive lock detect signal 226 from synthesizer 218 and amplifier alarm signal 114 from high-power amplifier 110 and (2) generate control signals 230 for voltage regulators and switches 232, which in turn generate signals to control the gains of amplifiers 210, 212, and 220. In addition, transmit up-converter alarm signal 116 is fed back from logic circuits 228 to alarm control board 118.
In telecommunication systems, it is important to limit intermodulation distortion (IMD) (also known as spectral regrowth (SR) in CDMA systems). Intermodulation distortion is a key performance-degrading effect in an amplifier, because it causes interference to adjacent channels that cannot be filtered out. IMD is a non-linear effect that is caused by the amplifier""s input power-output power characteristics being non-linear rather than linear. The non-linear characteristic is due to the physical properties of the semiconductor material used to fabricate the power transistors used in the amplifier. To get a linear input-output characteristic, some form of linearization, such as feed-forward and/or predistortion, is typically used, but it still usually does not provide optimum performance over a wide frequency range.
Thus, although an amplifier can be sufficiently optimized over a specific narrow frequency range (e.g., up to about a 20-MHZ frequency range), it is difficult to optimize a single amplifier to provide low levels of IMD across the entire 60-MHZ frequency range that PCS base station forward-link transmitters need to be able to support. One option is to design amplifiers, such as high power amplifier 110 of FIG. 1, to be so-called smart amplifiers that have complicated circuitry (e.g., frequency detection circuitry) that measures the frequency of the received signal, such as PCS block signal 108 of FIG. 1, and then automatically optimizes the operations of the amplifier accordingly based on that measured frequency. Unfortunately, such an option is often cost prohibitive for implementation in the base stations of typical PCS telecommunication systems. As a result, typical base station amplifiers, such as base station transmitter circuit 100 of FIG. 1, are optimized during manufacturing as best as possible for the entire 60-MHZ PCS forward-link frequency range, and then no optimization is performed when the base station is subsequently configured in the field (when the particular PCS frequency block for the base station is first known).
The present invention is directed to a transmitter circuit with frequency self-optimization for use in base station forward-link transmitters of wireless telecommunication systems conforming, for example, to the U.S. PCS standard. The self-optimizing base station transmitter circuit of the present invention uses digital information, already available in conventional base station transmitter circuits after they are configured for operation in the field, that indicates the specific PCS frequency block assigned to the base station, to optimize amplifier operations based on the specified PCS frequency block. As a result, the self-optimizing base station transmitter circuit can provide better performance (e.g., lower intermodulation distortion) over the entire 60-MHZ PCS frequency range than that provided by conventional base station transmitter circuits, without requiring the use of expensive smart amplifiers within the transmitter circuit. Moreover, since the present invention relies on the actual digital information used to indicate the specific PCS frequency block, the present invention may even be more accurate than transmitter designs that rely on smart amplifiers to detect block frequency. In addition, because the present invention reduces intermodulation distortion, the present invention can be used to increase system capacity (e.g., more CDMA codes per PCS frequency channel and/or greater base station coverage areas) by enabling higher transmit power levels without violating FCC rules regarding interference with transmissions in other frequency bands.
In one embodiment, the present invention is a transmitter circuit designed to operate over a specified transmit frequency range, comprising (a) an oscillator configured to generate a first signal at a first frequency; (b) an up-converter configured to convert the first signal into a second signal at a second frequency different from the first frequency, wherein the second frequency falls within the specified transmit frequency range and is specified by frequency information available within the up-converter; and (c) an amplifier configured to amplify the second signal. The up-converter generates a control signal for the amplifier based on the frequency information used to specify the second frequency, and the amplifier uses the control signal to automatically optimize its operations for amplifying the second signal.
In another embodiment, the present invention is an amplifier for use in a transmitter circuit designed to operate over a specified transmit frequency range, wherein the amplifier is configured to (1) receive and amplify an input signal having an input frequency within the specified transmit frequency range, (2) receive a control signal corresponding to the input frequency of the input signal, and (3) use the control signal to automatically optimize its operations for amplifying the input signal.
In yet another embodiment, the present invention is a method for generating signals within a transmitter circuit designed to operate over a specified transmit frequency range, comprising the steps of (a) generating a first signal at a first frequency; (b) converting the first signal into a second signal at a second frequency different from the first frequency, wherein the second frequency falls within the specified transmit frequency range and is specified by frequency information available within the transmitter circuit; (c) generating a control signal based on the frequency information used to specify the second frequency; and (d) amplifying the second signal with an amplifier, wherein the amplifier receives and uses the control signal to automatically optimize its operations for amplifying the second signal.